Tall oil is a major by-product of the kraft or sulfate pulping process employed in the manufacture of paper. In such a process, rosin acids and fatty acids which occur in pine wood as free acids or their esters are saponified in the black cooking liquor to their corresponding sodium salts. Black liquor separated from the cellulose portion of the wood in a kraft pulping process contains, among other things, sodium sulfite, sodium sulfide, about 8% sodium salts of lignin materials, fatty acid soaps, rosin acid soaps, and unsaponifiable organic compounds. The soaps which separate from the aqueous phase at a stage during evaporation of water from the black liquor float on the partially concentrated black liquor to form a layer of soap skimmings. The recovered skimmings are treated with sulfuric acid to convert the soap skimmings to the free carboxylic acid form known as crude tall oil.
In addition, the soap skimmings carry with them some black liquor, as a result of incomplete separation, and this black liquor contains lignin salt a lower molecular weight fraction of which, after acidification, will dissolve in the crude tall oil acids. Thus, black liquor left in the soap contributes to the lowering of the crude tall oil quality and its acid number. Acid number is a measure of the free resin acids and fatty acids available and recoverable, and expressed in the number of milligrams of KOH per gram of sample needed to neutralize the same.
Crude tall oil is a dark brown mixture of fatty acids, rosin, and neutral materials. The fatty acids are primarily a mixture of unsaturated oleic and linoleic acids and saturated palmitic and stearic acids. Rosin is composed of resin acids. Crude tall oils are fractionally distilled to separate them into heads (low boiling fractions), tall oil fatty acids, distilled tall oil (mixed fatty and resin acids), rosins, and pitch (residue).
The tall oil fractionally distilled product collected from the mid-cut fractions of the crude tall oil, i.e., the tall oil fatty acids, distilled tall oil, and rosins, are widely employed as emulsifiers in polymerization of rubber. After distillation, the mid-cut fractions of tall oil are sometimes disproportionated, as with an iodine catalyst, by heating at high temperature for one or more hours, typically at 485.degree. F. for about 1 to 2 hours. Disproportionation involves the step of heating fractions of the crude tall oil in the presence of certain catalysts to produce a molecular double-bond rearrangement in the labile abietic-type acids and linoleic fatty acid to convert them into stable hydrogenated and dehydrogenated acids. Disproportionation catalysts include sulfur dioxide, iodine, nickel, and palladium. The disproportionated tall oil undergoes less oxidation in end use applications in that the conjugated double bond systems of the abiatic-type acids are removed. After disproportionation, the tall oil mid-cut fraction is steam sparged at a temperature of around 440.degree. F. to remove iodine and certain trace amounts of lignin present therein.
The amount of lignin in crude tall oil has been analyzed to be on the order of 0.5-4%. This lignin is sufficiently low in molecular weight to be soluble in the crude tall oil and its volatility is such that it distills over in the same boiling ranges as the various tall oil fractions. It is known from the literature that lignin decomposes more than it is polymerized at temperatures of between about 300.degree. F. to 400.degree. F., whereas higher temperatures approaching 550.degree. F. promotes lignin polymerization over degradation.
The presence of lignin in tall oil emulsifiers used in emulsion polymerization of rubber or other like polymers is deleterious to both the polymerization rate and latex quality. Lignin makes the polymerization reaction particularly sensitive to traces of oxygen that are present in commercial continuous emulsion polymerization reactor systems. Such sensitivity causes a reduction in polymerization rate in the presence of oxygen and also reduces production throughput unless extra initiator is utilized in the polymerization reaction. This reduction in polymerization rate is due to the lignin-oxygen impurities which terminate the growing rubber molecules in latex particles before they would normally terminate through interaction with a new free radical produced by the initiator. For this reason, polymer chains terminated by lignin-oxygen impurities are, on the average, lower in molecular weight than normal. This creates an undesirable degree of stickiness in rubber, together with other detrimental effects in the physical properties of rubber.
In a present method of producing tall oil latex emulsifiers, the iodine catalyst employed in the disproportionation step decomposes the lignin present in the feed stock to low enough molecular weight that it is removed from the product along with the iodine during the steam sparge after disproportionation. However, to satisfactorily destroy this lignin and prevent its carry-over into the final product, it has been necessary to carry out the disproportionation reaction at such a high temperature, e.g., 485.degree. F., that a portion of the product is decarboxylated. There is also some degradation of color in the product at this temperature. These factors adversely effect the quality of the rubber products produced using the tall oil emulsifiers in the polymerization reaction.